This invention relates to a coupling for airport surveillance antenna systems and other rotating structural systems and subsystems.
Airport surveillance antennas include a rotating antenna mounted to a rotating part of an antenna pedestal. The antenna pedestal houses a rotary joint on its nominal center of rotation. The upper section of this rotary joint rotates with the antenna, and its lower section must not rotate with the antenna because it houses one or more encoders which determine the antenna pointing angle. If the lower section of the rotary joint rotates, the encoders rotate as well which results in antenna azimuth angle errors.
Because of inherent small offsets between the pedestal rotation axis and the rotary joint rotation axis, component coefficient of thermal expansion differences, and non-parallelism of the pedestal and rotary joint bearings, however, the lower section of the rotary joint cannot be clamped in or fixed rigidly with respect to the structure of the antenna pedestal.
Instead, the lower section of the rotary joint must be allowed to follow a small circular orbit inside the antenna pedestal to prevent wear or destruction of the pedestal and rotary joint bearings and/or wear or destruction of components and structure inside the antenna pedestal. Thus, the lower section of the rotary joint must be allowed to translate in three directions and tilt or rotate about two axes, but not rotate about the axis about which the antenna rotates or axes parallel thereto. Thus, five degrees of freedom of movement must be allowed for the lower section of the rotary joint, but not the sixthxe2x80x94rotation with the rotating antenna.
Previously, the lower section of the rotary joint was constrained against rotation with the antenna by a simple tangential link with ball joints on each end connecting the lower section of the rotary joint to a fixed point on the antenna pedestal. This tangential link allowed the necessary five degrees of freedom of movement of the lower section of the rotary joint but the tangential link also allowed an undesirable once per antenna revolution back and forth rotation of the lower section of the rotary joint. This periodic rotary motion moved the encoder bodies together with the lower section of the rotary joint resulting in a direct error in the azimuth angle of the antenna reported by the encoders.
Attempts to reduce this error were only minimally successful and even after such efforts the reported error was still roughly equal to the whole error budget in a Digital Airport Surveillance Radar (DASR).
Moreover, the asymmetry of this single tangential link imposed a side load on the rotary joint proportional to the rotary joint frictional torque. This side load plus other loads from the waveguide and electrical cable connections housed by the rotary joint deflect the rotary joint laterally adding to the rotary joint bearing loads and azimuth angle error.
It is therefore an object of this invention to provide a coupling for the rotary joint of an antenna system which reduces and virtually eliminates encoder errors.
It is a further object of this invention to provide such a coupling which is inexpensive, simple in design, and simple in construction.
It is a further object of this invention to provide such a coupling which prevents rotation of the lower section of the rotary joint with the antenna but yet which still allows the lower section of the rotary joint to translate and rotate as required in five other degrees of freedom of movement.
It is a further object of this invention to provide such a coupling which does not allow crosstalk which would result in rotation of the lower section of the rotary joint and such a coupling which does not contribute to side or bearing loads.
It is a further object of this invention to provide a coupling which can be used in connection with systems other than antenna pedestals: for example, telescope polarization drive subsystems.
This invention results from the realization that azimuth angle errors, side load torque, backlash, and crosstalk caused by rotation of the lower section of the rotary joint of an antenna pedestal can be reduced and yet the lower section of the rotary joint rendered displaceable in five other directions by the addition of a low profile, large through-hole, ring member surrounding the rotary joint which itself is translatable in only a first direction and by attaching the lower section of the rotary joint to the ring member in a way that relative movement is possible between the ring member and the rotary joint in all directions except in the first direction and rotation about the ring axis.
Two inexpensive steel cables attach the lower section of the rotary joint to the ring member preventing rotation of the lower section of the rotary joint about the ring axis to eliminate encoder errors, but the cables allow translation of the lower section of the rotary joint in the direction of the ring axis and in the direction of an axis orthogonal to the longitudinal axis of the cables to compensate for thermal mismatches and bearing offsets, respectively. The cables, however, do not elongate or contract by any appreciable amount along their longitudinal axes. Therefore, the ring member itself and thus the lower section of the rotary joint is allowed to move in the direction of the longitudinal axes of the cables by connecting the ring member to the antenna housing by steel flexure plates which only allow translation of the ring member in the direction of the longitudinal axes of the cables but restrict translation of the ring member in the direction of all other axes and also restrict tilting (i.e., rotation) of the ring member about all three axes.
This invention features a coupling comprising a ring member attached to a housing, the ring member having a through-hole and a ring axis; a body connected to the ring member by a pair of yielding but axially inextensible members preventing rotation of the body about the ring axis and preventing movement of the body in the direction of the axes of the inextensible members; and means for allowing displacement of the ring member with respect to the housing in the direction of the axes of the inextensible members to thereby allow the body five degrees of freedom of movement with respect to the housing but not rotation about the ring axis.
The ring member is typically round and has a diameter greatly exceeding its thickness for low profile applications. In the preferred embodiment, the inextensible members are cables. Each of the cables include on one end a clamping block fixed to the ring member. Each of the cables typically also include on an opposing end a reaction arm fixed to the body. The axes of the cables preferably extend in a first direction orthogonal to the ring axis and the reaction arms then preferably extend in a second direction orthogonal to the ring axis and orthogonal to the first direction.
The means for allowing displacement of the ring member with respect to the housing may include flexure members attached on one end to the ring member and attached on an opposing end to the housing. The flexure members may be thin flat steel plates diametrically opposed and attached to the ring member along an axis parallel to the axes of the inextensible members. The inextendible members are also diametrically opposed from each other about the ring member.
In the preferred embodiment, the housing is a fixed portion of an antenna mount, and the housing includes a cavity in which the ring is disposed. The body is the non-rotating portion of a rotary joint. At least one encoder is usually attached to the non-rotating portion of the rotary joint.
A coupling in accordance with this invention includes a ring member attachable to a housing, the ring member having a through-hole and a ring axis; a pair of yielding but axially inextensible members each connected on one end to the ring member and each connectable on an opposing end to a body to be constrained against rotation about the ring axis; and a pair of flexure members flexible in the direction of the axes of the inextensible members, each flexure member connected on one end to the ring member and connectable on an opposing end to the housing.
The ring member is typically round and has a diameter greatly exceeding its thickness for low profile applications. The axially inextensible members may be cables each of which include on one end a clamping block fixed to the ring members and, on an opposing end, a reaction arm.
The axes of the cables extend in a first direction orthogonal to the ring axis and the reaction arms extend in a second direction orthogonal to the ring axis and orthogonal to the first direction. The flexure members are usually diametrically opposed and attached to the ring member along an axis parallel to the axes of the inextensible members. The inextensible members are diametrically opposed on the ring member along an axis orthogonal to the axis of the flexure members.
This invention also features a coupling comprising a housing; a ring member disposed in the housing, the ring member having a through-hole; a first four bar linkage attaching the ring member to the housing; a body; and a second four bar linkage attaching the ring member to the body. Two bars of the second four bar linkage include flexible but axially inextensible members.
The first four bar linkage typically includes spaced flexure plates each connected on one end to the housing and each connected on an opposing end to the ring member. The flexible but axially inextensible members of the second four bar linkage typically include cables connected on one end to the ring member and connected on an opposing end to the body.
The cables are each connected to the body via a reaction arm. The housing may be a fixed portion of an antenna mount, the housing includes a cavity, and the ring member is disposed in the cavity. The body is then the non-rotating portion of a rotary joint and may include encoders attached to it.